This was a continuation of a project initiated in Jerry Crabtree's laboratory (Laboratory of Pathology, DCBD, NCI). There I found that a portion of an exon in the human gamma fibronogen was duplicated approximately 1 kbp away in an adjacent intron. The duplication was flanked by a long inverted repeat sequence (IR) which was found to be single copy by Southern blot analysis. The structure of this duplication was reminiscent of transposable elements in lower eukaryotes and prokaryotes and had some sequence homology with the FB transposable elements in Drosophilia. Since 1-2% of most eukaryotic genomes consist of single copy IR, this may represent a major type of genetic duplication. Recently, a second single copy IR was identified in the murine immunoglobulin locus. By sequence analysis, I found that it also flanked a region which was duplicated 1 kbp away. Thus, it appears likely that many or most single copy IR in mammalian genomes are probably associated with regional genetic duplications. Therefore, it is possible that this type of genetic duplication constitutes a major type of regional duplication in eukaryotic evolution.